Method for making a low density multi-ply paperboard with high internal bond strength

ABSTRACT

Methods for improving the internal bond strength of paperboard with greater than 25 percent crosslinked fiber in at least one ply are described. In the methods, additives are added to the slurry in various combinations and order while maintaining the ionic demand of the slurry at less than zero. Paperboard with high ZDT, Scott Bond and Taber Stiffness is obtained.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims priority to U.S. Application No. 60/664,440, filed Mar. 22, 2005.

FIELD

The present application relates to increasing the bond strength in a multi-ply paperboard that has high crosslinked cellulose fiber present in at least one of the plies.

SUMMARY

This application is directed to improving the internal bond strength of paperboard with greater than 25 percent crosslinked fiber in at least one ply. In the method, additives are added to the slurry in various combinations and order while maintaining the ionic demand of the slurry at less than zero. Paperboard with high ZDT, Scott Bond and Taber Stiffness is obtained.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows the effect of various chemical additions on ionic demand of a 3-3.2 percent slurry;

FIG. 2 shows the effect of chemical addition on ionic demand of mid-ply simulation using polyacrylic acid crosslinked cellulosic fibers;

FIG. 3 shows the effect of chemical addition on ionic demand of mid-ply simulation using citric acid crosslinked cellulosic fibers.

FIG. 4 is a schematic representation of the machine chest and additive entry points suitable for carrying out the process in the present application;

FIG. 5 represents a multi-ply paperboard machine suitable for a multi-ply product;

DESCRIPTION

In single or multi-ply paperboard where the inner plies contain greater than approximately 25 percent crosslinked cellulose fiber the density of the stratum will drop below 0.4 g/cc. As a result, the internal bond strength can drop so low as to not only be well below levels required for converting the paperboard into packaging products but also below the level where conventional methods of increasing the internal strength cannot provide enough increase to meet minimum levels needed for converting. The present application provides a method for increasing the internal bond of low density paperboard back into the range which is useable for converting.

In this application, use of high concentrations of wet end additives have been demonstrated while producing low density paperboard.

A distinguishing characteristic of the present application is that at least one ply of the paperboard, whether a single-ply or a multiple-ply structure, contains crosslinked cellulose fibers and strength enhancing additives such as anionic and cationic starches to offset the board strength lost by adding the crosslinked cellulosic fibers. The crosslinked cellulosic fibers increase the bulk density of the insulating paperboard characteristics of the board. The paperboard also contains chemical pulp fibers. As defined herein chemical pulp fibers useable in the present application are derived primarily from wood pulp. Suitable wood pulp fibers for use with the application can be obtained from well-known chemical processes such as the kraft and sulfite processes, with or without subsequent bleaching. Softwoods and hardwoods can be used. Details of the selection of wood pulp fibers are well known to those skilled in the art. For example, suitable cellulosic fibers produced from southern pine that are useable in the present application are available from a number of companies including Weyerhaeuser Company under the designations C-Pine, Chinook, CF416, FR416, and NB416. A bleached Kraft Douglas Fir pulp, KKT, Prince Albert Softwood and Grande Prairie Softwood, all manufactured by Weyerhaeuser are examples of northern softwoods that can be used. Mercerized fibers such as HPZ and mercerized flash dried fibers such as HPZ III, both manufactured by Buckeye Technologies, Memphis Tenn., and Porosinier—J-HP available from Rayonier Performance Fibers Division, Jessup, Ga. are also suitable for use in the present application when used with crosslinked cellulose fibers. Other non crosslinked cellulose fibers include chemithermomechanical pulp fibers (CTMP), bleached chemithermomechanical pulp fibers (BCTMP), thermomechanical pulp fibers (TMP), refiner groundwood pulp fibers, groundwood pulp fibers, TMP (thermomechanical pulp) made by Weyerhaeuser, Federal Way, Wash., and CTMP (chemi-thermomechanical pulp) obtained from NORPAC, Longview, Wash., sold as a CTMP NORPAC Newsprint Grade, jet dried cellulosic fibers and treated jet dried cellulosic fibers manufactured by the Weyerhaeuser Company by the method described in U.S. application Ser. No. 10/923,447 filed Aug. 20, 2004. These fibers are twisted kinked and curled. Additional fibers include flash dried and treated flash dried fibers as described in U.S. Pat. No. 6,837,970,

Suitable crosslinking agents for making crosslinked fibers include carboxylic acid crosslinking agents such as polycarboxylic acids. Polycarboxylic acid crosslinking agents (e.g., citric acid, propane tricarboxylic acid, and butane tetracarboxylic acid) and catalysts are described in U.S. Pat. Nos. 3,526,048; 4,820,307; 4,936,865; 4,975,209; and 5,221,285. The use of C₂-C₉ polycarboxylic acids that contain at least three carboxyl groups (e.g., citric acid and oxydisuccinic acid) as crosslinking agents is described in U.S. Pat. Nos. 5,137,537; 5,183,707; 5,190,563; 5,562,740; and 5,873,979.

Polymeric polycarboxylic acids are also suitable crosslinking agents for making crosslinked fibers. These include polymeric polycarboxylic acid crosslinking agents are described in U.S. Pat. Nos. 4,391,878; 4,420,368; 4,431,481; 5,049,235; 5,160,789; 5,442,899; 5,698,074; 5,496,476; 5,496,477; 5,728,771; 5,705,475; and 5,981,739. Polyacrylic acid and related copolymers as crosslinking agents are described U.S. Pat. Nos. 5,549,791 and 5,998,511. Polymaleic acid crosslinking agents are described in U.S. Pat. No. 5,998,511 and U.S. application Ser. No. 09/886,821. CHB405, a citric acid crosslinked cellulose fiber and CHB505, a polyacrylic acid crosslinked cellulose, both commercially available from Weyerhaeuser Company, Federal Way, Wash. were used in this work.

In single or multi-ply paperboard construction a mixture of wood pulp fibers and crosslinked cellulose fibers are used. In one embodiment the crosslinked cellulosic fibers are present in at least one layer at a level of 25 to 80 percent by total fiber weight of the ply. In another embodiment the crosslinked fibers are present at a level of 40 to 75 percent by total fiber weight of the ply and in yet another embodiment they are present at a level of 50 to 70 percent by total fiber weight of the ply.

Single-ply handsheets designed to simulate the mid-ply of low density multi-ply paperboard were made. A 0.015 percent to 0.035 percent consistency slurry was used in these studies. The handsheet making equipment was a standard 8″×8″ sheet mold modified with an extended headbox so that twice the normal volume of stock was used. This modification was necessary to improve handsheet formation when using materials designed to generate high bulk such as crosslinked cellulosic fibers. Fiber weights are expressed as a weight percent of the total fiber dry weight; additives are based on weight of dry fiber.

A series of handsheets were made using different levels of wet-end additives and different addition order to demonstrate the level of internal bond strength that could be generated by starch loading the web. The additives were added to the slurry in the order across each sample row and the slurry stirred after each addition. Avebe® AP25, an anionic starch, was obtained from Carolina Starches, North Charleston, S.C., Stay-Lok®300 and Stay-Lok®330, both cationic starches, were obtained from A.E. Staley, a subsidiary of Tate and Lyle, Decatur, Ill. Kymene® was obtained from Hercules, Wilmington Del. and RediBOND® 3050 from National Starch, Indianapolis Ind. Each handsheet was then coated with Polyvinyl Alcohol (PVA) coating, Celvol V24203 supplied by Celanese Ltd., Houston, Tex. The total coat weight was about 50 g/m² and was divided equally to each side of the sheet. The coating was added to the surface to facilitate testing Z-direction tensile (ZDT) and Internal Scott Bond, because low density structures without the coating tended to separate at the tape instead of the within the sheet. Each sheet was evaluated for several physical properties including Basis weight, ZDT, Internal Scott Bond and Taber Stiffness (15°). Table I below shows the results for these key characteristics. TABLE I Handsheet formulation and addition order Sta- Sta- Target Avebe ® Loc ® Loc ® ® Basis Scott Taber Sample BW, CHB405, D. Fir, AP25*, 300, Kymene ®, 300, Aquapel ®, Wt., Density, ZDT, Bond, Stiffness, No. g/m² wt. % wt. % #/t #/t #/t #/t #/t g/m² g/cm³ kPa J/m² g cm 1 250 60 40 0 0 5 25 4 301 0.264 40 67 364 2 250 60 40 40 20 5 20 4 306 0.262 85 113 433 3 250 60 40 80 20 10 10 4 305 0.273 204 119 403 D. Fir, Douglas Fir

ZDT (Z-direction tensile) strength was determined by TAPPI method T 541 om-05, Scott Bond strength was determined by TAPPI 569 om-00 and Taber Stiffness by TAPPI T 489 om-04.

Sample 1 is a fiber formulation designed to deliver low density paper and uses conventional wet chemistry. The result is a very low ZDT and Scott bond. Samples 2 shows that by adding 4% total starch to the furnish, the ZDT and Scott Bond essentially double. Samples 3 shows that when the amount of anionic starch is doubled, the cationic starch remains constant and Kymeme® 557H, a higher ionic demand cationic polymer (+2.2 meq/g) than the cationic starch, (+0.3 meq/g, Table III), is used to balance the additional anionic charge, the result is a further increase in internal bond, increasing ZDT by 500% over the control; Scott Bond is nearly doubled.

The data show that anionic starch in combination with a cationic fixative allows for effective retention of starch in the wet end. As defined herein, a fixative is a charged polymer that ionically bonds to a molecule of the opposite charge by an ionic bond. Examples of fixatives include, but are not limited to cationic starch, polyamines, polyaluminum chloride® (PAC), poly DADMAC (polydiallyldimethylammonium chloride), EPEDMA, Kymene®, PAAE (polyamidoamine-epichlorohydrin), PASS (polyaluminiumsilicasulate), PEI (polyethyleneimine), and cationic polyacrylamide.

In one embodiment a cationic fixative is added to the slurry followed by addition of an anionic starch and then followed by addition of a cationic starch. In each case, after addition of the additive, the ionic demand of the slurry is less than zero. In another embodiment the cationic starch is added to the slurry before the anionic starch and the cationic fixative is added after the anionic starch. In yet another embodiment a cationic fixative is added to the slurry and the anionic starch is added to the slurry subsequent to addition of the cationic fixative. In each case, after addition of the additive, the ionic demand of the slurry is less than zero. In a further embodiment the anionic starch is added to the slurry before addition of the cationic fixative, in each case, after addition of the starch or fixative, the ionic demand of the slurry is less than zero. A combination of anionic and cationic starches can also be used. In one embodiment a cationic starch is added to the slurry followed by addition of an anionic starch, in each case, after addition of the starch, the ionic demand of the slurry is less than zero. In yet another embodiment the anionic starch is added to the slurry followed by addition of the cationic starch. After addition of the starch the ionic demand of the slurry is less than zero.

In one embodiment Kymene® is added at a level of from 2.5 lb/t to 20 lb/t. In another embodiment it is added at a level of 5 lb/t to 15 lb/t and in yet another embodiment it is added at a level of 8 lb/t to 10 lb/t.

In general the impact of the starch loading on taber stiffness at 15° bend is small. For single ply handsheets this is reasonable because caliper is the dominate variable effecting bending stiffness. The impact of the starch loading on density is small enough that the increase in elastic modulus of the sheets due to the starch loading compensates for the small changes in caliper. In a multi-ply web the same response is expected.

Comparison of Different Anionic and Cationic Starches

Table II shows the basic formulations and conditions used in making handsheets in this study. TABLE II Handsheet Formulation, Addition Order and Sheet Properties Basis Aniofax ®, Redibond Sta-Lok ® Sta-Lok ® Basis Scott Taber Sample Wt., CHB505 Douglas AP25, ®3050 300, 330 Wt., Density, ZDT, Bond, Stiffness, No. g/m² wt. % Fir, wt. % #/t #/t #/t #/t g/m² g/m² kPa J/m² g cm 4 250 60 40 0 0 20 0 302 0.258 36 96 334 5 250 60 40 40 0 20 0 303 0.263 60 89 325 6 250 60 40 0 40 20 0 307 0.254 53 100 351 7 250 60 40 80 0 40 0 307 0.263 84 93 378 8 250 60 40 0 80 40 0 310 0.264 72 110 350 9 250 60 40 40 0 0 20 308 0.257 57 102 368 10 250 60 40 0 40 0 20 311 0.247 51 111 378 All samples had 5 lb/t Kymene ® added before the anionic starch addition and 4.5 lb/t Aquapel ® added after the cationic starch, Sta-Loc ®330, addition.

In these studies both the addition level of anionic and cationic starches was changed as well as the ionic demand of the starch. Ionic demand refers to the number of charges (both positive or negative) needed to bring the net charge of a given volume of furnish to zero. Cationic demand refers to the quantity of cationic charges needed to bring the sample to a net zero charge. A sample with a cationic demand is net anionic. Anionic demand refers to the quantity of anionic charges needed to bring the sample to net zero charge. A sample with an anionic demand is net cationic.

Aniofax® AP25 (−0.25 meq/g ionic demand) was exchanged with Hercules RediBOND® 3050 which has a lower ionic demand of ˜−0.19 meq/g. Further the Sta-Loc® 300, with an ionic demand of +0.30 meq/g was exchanged for Sta-Lok® 330 having an ionic demand of about +0.41 meq/g. The results shown in Table II indicate that an equal mass of the RediBOND® 3050 as Aniofax® AP25 results in about 10% lower ZDT. This decrease is believed to be due to the lower charge contribution of the RediBOND® 3050 resulting in a lower retention of the subsequent addition of the starch. However, the Scott Bond and Taber Stiffness development are approximately equivalent (Sample no. 5 vs. 6, 7 vs. 8 and 9 vs. 10).

Ionic Demand Balance

The technology relies on the ability to balance the ionic demand in the wet end of the paper machine such that 1) anionic polymeric materials can be retained on the fibers and fines without excess remaining in the water system, 2) the fibers and system do not pass through the zero charge point which destabilizes retention and drainage 3) since pulp fibers are anionic, some cationic material can be added, however, adding too much cationic material without balancing the excess anionic demand will either cause the fibers to flocculate reducing formation and/or cause the drainage to drop, impacting the runnability.

Each of the components used in the paperboard containing crosslinked fiber in this disclosure has a specific charge density typically measured by ionic demand titration. A Mutek PCD-Titrator was used for the particle charge titration coupled with the PCD 02 Particle Charge Detector for measuring the ionic demand of the component or fiber furnish. The method was performed according to a procedure from A.E. Staley Manufacturing, a subsidiary of Tate and Lyle, Decatur, Ill. The method is as follows.

1. Turn the Mutek on using the power switch on the back of the instrument.

2. Place 10 mL of a well mixed sample in the sample vessel. Insert the plunger and washer into the vessel. The sample consistency should be no more than 0.83. Thick stock samples should be diluted.

3. With the instrument turned on, the plunger should move up and down and a mV potential should be displayed. The sign of the potential (+ or −) indicates whether the sample is cationic (+) or anionic.(−).

4. Titrate the sample with the appropriate titrant until the mV potential reads 0 mV (PolyDADMAC is the cationic polymer and is used to titrate anionic samples; PVSK or PESNa are the anionic polymers used to titrate the cationic samples). A buret or syringe can be used to deliver the titrant to the sample. Titration should not be conducted with no more than 4 mL of titrant since higher volumes will give inaccurate measurements. If the sample requires more than 4 mL of titrant, the sample should be diluted or more concentrated titrant should be used.

5. Record the amount of titrant used to titrate the sample. To calculate the demand of the system, use the following equation: “Ionic Demand” (ueq/L)=(mL titrant)×(% titrant dilution)×(sample dilution)

Specific components whose ionic demand were measured by the Mutek method are noted in Table III, other component values are from suppliers. TABLE III Ionic Demand Of Specific Components Component Ionic Demand, meq/g Fully Bleached Softwood Pulp (a) −0.015, total (a) Fully Bleached Softwood Pulp* −.0015, available (a) CHB505 −0.63, total (a) CHB505 −0.017, available (a) CHB 405 −.0.43 total (a) CHB405 −0.015, available (a) Kymene ®577H +2.2 Hercobond ® 2000 −1.8 Sta-Loc 300 +0.3 RediBOND ®3050 (HRB ®3050) −0.19 Aniofax AP25 −0.25 Sta-Lok ® ®330 +0.41 PSM -Particol ®BX — Aquapel ®650  0 (a) determined by Mutek method; other values were obtained from suppliers.

With reference to the table, the difference between the total and available ionic demand represents the amount of charge that is internal to the fiber that is not accessible to polymers of molecular weight above 300,000 g/mole. For papermaking, the available ionic demand is more representative of the results obtained in practice than the total ionic demand.

The situation is further complicated in a paper machine wet-end where dilution water from outside sources and/or wash water from pulp mill bleaching stages contain ionic materials, (both dissolved and dispersed), is used to control consistency of the pulp slurry. In integrated mills where excess ionic materials are present, materials added to the pulp slurry to increase internal bond strength can be consumed by the excess ionic materials. Also, the available ionic sites on pulp will also depend on how much refining has been done and on the basic fiber morphology, i.e. the smaller the fiber or partial fiber the higher available surface area, and therefore the higher available ionic demand.

In general it may be stated that the fiber slurry is anionic to start with and should remain anionic through the paper making process i.e. the ionic demand of the slurry should less than zero.

Calculated Balance

For the purpose of this work the ionic demand for formulations listed in Tables I and II were calculated using the ionic demand values in Table III and the ionic material in the slurry, other than the materials listed, was ignored.

The addition order of ionic additives and the amount should dictate the ionic demand of the stock furnish at any time during the forming process. For these experiments the estimated ionic demand is shown in Tables IV and V. TABLE IV Calculated Cumulative Ionic Demand Of The Slurry At Each Addition Step In Table I Anionic Anionic Starch Cationic Starch Cationic Avebe Starch Avebe Starch Aniofax ® Sta-Lok ® Aniofax ® Kymene ® Sta-Lok ® Aquapel ® At Sample CHB405 D. Fir Pulp Stock AP25 300 AP25 557H 300 650 forming No (ueq/kg) (ueq/kg) (ueq/kg) (ueq/kg) (ueq/kg) (ueq/kg) (ueq/kg) (ueq/kg) (ueq/kg) (ueq/kg) 1 −9000 −600 −9600 −4100 −350 −4350 −350 5500 3750 2 −9000 −600 −9600 −15600 −11600 −6100 −3100 −3100 −3100 −5000 3000 5500 3000 3 −9000 −600 −9600 −19600 −16600 −5600 −3600 −3600 −3600 −10000 3000 11000 1500 Note: All additions are on an oven dry basis. The second entry in a cell is the ionic demand contribution by the component

TABLE V Calculated Cumulative Ionic Demand Of The Slurry At Each Addition Step In Table II Avebe Starch Starch Aniofax ® Hercules Sta-Lok ® Sta-Lok ® Aquapel ® At Sample CHB505 D. Fir Pulp Stock Kymene ® AP25 RediBOND ® 300 330 650 forming No ueq/kg ueq/kg ueq/kg ueq/kg ueq/kg ueq/kg ueq/kg ueq/kg ueq/kg ueq/kg 4 −10200 −600 −10800 −5300 −2300 −2300 −2300 5500 3000 5 −10200 −600 −10800 −5300 −10300 −7300 −7300 −7300 5500 −5000 3000 6 −10200 −600 −10800 −5300 −9100 −6100 −6100 −6100 5500 −3800 3000 7 −10200 −600 −10800 −5300 −15300 −9300 −9300 −9300 5500 −10000 6000 8 −10200 −600 −10800 −5300 −12900 −6900 −6900 −6900 5500 −7600 6000 9 −10200 −600 −10800 −5300 −10300 −6200 −6200 −6200 5500 −5000 4100 10 −10200 −600 −10800 −5300 −9100 −5000 −5000 −5000 5500 −3800 4100 Note: 1 kg of oven dry furnish was used as the basis. The second entry in a cell is the ionic contribution by the component

TABLE VI Measured Ionic Demand Of Mid-Ply Stock After Various Chemical Additions Starch Starch PMA Cross- Sta- Aqua- Pulp Sta- Aqua- HRB ® Part- At link D. Ky- HRB ® Lok ® pel ® HRB ® Stock Ky- HRB ® lok ® pel ® 3050 icol Form- Sam- Fiber, Fir mene ® 3050, 300 650, 3050, ueq/ mene ® 3050, 300 650 ueq/ BX ing ple (%) (%) (#/t) (lb/t) (lb/t) (%) (lb/t) mL ueq/mL ueq/mL ueq/mL ueq/mL mL ueq/mL ueq/mL 11 60 40 4.5 15 4.5 −305 −121 −68 −68 −68 12 60 40 9 60 4.5 40 −124 −40 385 385 195 195 13 60 40 9 40 60 4.5 −154 0 −359 −29 −29 −29 14 60 40 9 27 4.5 40 −120 −43 167 167 −33 −33 15 60 40 9 40 27 4.5 −150 −17 −325 −27 −27 −27 16 60 40 9 40 27 4.5 −191 −40 −298 −211 −211 −191 −191 11 60 40 4.5 15 4.5 −180 −118 0 0 0 11 60 40 4.5 15 4.5 −157 −83 −28 −28 −28 15 60 40 9 40 27 4.5 −208 −22 −453 −412 −412 −412 15 60 40 9 40 27 4.5 −168 −19 −444 −222 −222 −222 16 60 40 9 40 27 4.5 −139 −49 −417 −249 −249 −222 −249 16 60 40 9 40 27 4.5 −160 −34 −563 −133 −133 −356 −356 17 60 40 4.5 15 4.5 −224 −153 −72 −72 −72 17 60 40 4.5 15 4.5 −269 −165 −70 −70 18 60 40 9 40 27 4.5 −351 −28 −542 −268 −268 −268 18 60 40 9 40 27 4.5 −286 −45 −504 −241 −241 −241 19 60 40 9 40 27 4.5 −297 −24 −468 −209 −209 −249 −249 19 60 40 9 40 27 4.5 −257 −34 −496 −223 −223 −271 −271

Table VI represents the measured amount of ionic demand of similar combinations of additives at a 0.5 percent slurry consistency using mill white water for dilution.

In the cases where the cationic material was all added prior to the additional anionic starch the ionic demand of the system crossed zero charge point, Table VI, 12 and 14 and FIG. 1. This is known to destabilize fines retention and impacts flocculation and drainage in the wet end of the machine, potentially resulting in a loss in ZDT. In FIG. 1, the addition points are as follows: A is pulp stock furnish, B is Kymene® addition, C is Hercules RediBOND® 3050 addition for 13, 15 and 16 and Sta-Loc®300 for 12 and 14 and D is Hercules RediBOND® addition. Samples 11-16 were made with CHB405 and samples 17-19 were made with CHB505. Sample 16 and 19 each had 0.5 lb/t PSM Particol®BX added after the addition of HRB®3050. Comparing samples made with CHB405, (citric acid crosslinked fibers) with CHB505 (polyacrylic acid crosslinked fibers), Table VI, 15 and 16 vs 18 and 19, FIGS. 2 and 3 indicates there is a small difference in ionic demand between the two crosslinked cellulose fibers grades. The net result is no specific impact on the number of available sites for retaining binding materials. This is supported by the fact that adding the same amount of Kymene® produces the same ionic demand.

A single or multi-ply board can be made using a 3-3.2 percent slurry consistency of a 50/50 dry weight ratio of a crosslinked fiber such as polyacrylic acid crosslinked fiber, (CHB505), and Douglas Fir fiber. Slurry consistencies from 0.05 percent to 4 percent can also be used in preparing the paperboard. The term “consistency” means the percent solids content of a solid and liquid mixture, for example, a two percent consistency means there are two grams of cellulose fibers in one hundred grams of fiber and liquid. Dry weight as defined herein means representative fibers are dried at 105° C.+/−2° C. and weighed at one hour increments until a constant weight is obtained. In practice, the moisture content of the commercial grade fibers is approximately 9 percent. The Douglas Fir can be refined to 500 CSF or lower prior to addition to the machine chest. A standard 4 mm barrier screen can be used before the headbox. Addition points of the various additives such as Kymene®, anionic and cationic starch are shown in FIG. 4. The paperboard formed can have a basis weight of 200 to 500 g/m² with surface plies of 15-25 percent by dry weight each of the total fiber and the center ply at 50-70 percent of the total fiber dry weight. In one embodiment a single ply paperboard is made. In another embodiment a paperboard with at least two plies is made. Crosslinked cellulose fiber can be in both plies or in only one. In yet another embodiment a paperboard can be made with three plies. The plies may all be the same, all different or there can be combination of the various plies.

Starch and other additives can be added at the locations shown in FIG. 4.

Surface plies can be formed separately from the mid-ply using conventional Fourdrinier technology with stock consistency ˜0.6 percent, as shown in FIG. 5.

The mid-ply can be formed using Beloit top former directly on the bottom-ply on the Fourdrinier table. Dewatering can be accomplished by a top wire drainage unit.

The top ply can be combined with the bottom and middle plies via a pick-up roll. The combined web can subsequently be processed by typical paper machine unit operations, wet pressing with conventional felted wet presses, steam can drying, size press where ˜2 to 2.5 percent starch based on dry weight fiber can be applied to the web, followed by steam can drying to 6.5 percent moisture, wet calendering, dry calendering and winding. 

1. A method for forming a paperboard comprising the steps of: forming a slurry of cellulose fibers comprising crosslinked fibers; adding a cationic fixative and mixing with said slurry; adding an anionic starch subsequent to adding said cationic fixative; adding a cationic starch subsequent to adding said anionic starch; wherein, after each addition step, the slurry ionic demand is less than zero; forming a fibrous web layer by withdrawing liquid from said slurry; drying said web to form a paperboard.
 2. The method of claim 1 wherein said crosslinked fibers are present at a level from 25 to 80 percent of the total fiber weight in at least one ply of the paperboard.
 3. The method of claim 2 wherein the crosslinked fibers are present at a level from 40 to 75 percent of the total dry weight of the fiber weight in at least one ply of the paperboard.
 4. The method of claim 1 wherein the total starch level is 50 to 120 lb/t.
 5. The method of claim 1 wherein the total starch level is 60 to 100 lb/t.
 6. The method of claim 1 wherein the total starch level is 80 to 90 lb/t.
 7. The method of claim 1 wherein the cationic starch is added to the furnish before the anionic starch and the cationic fixative is added after the anionic starch.
 8. A method for forming a paperboard comprising the steps of: forming a slurry of cellulose fibers comprising crosslinked fibers; adding a cationic fixative and mixing with said slurry; adding an anionic starch subsequent to adding said cationic fixative; wherein, after each addition step, the slurry ionic demand is less than zero; forming a fibrous web layer by withdrawing liquid from said slurry; drying said web to form a paperboard.
 9. The method of claim 8 wherein the anionic starch is added to the slurry before the cationic fixative.
 10. The method of claim 8 wherein said crosslinked fibers are present at a level from 25 to 80 percent of the total fiber weight in at least one ply of the paperboard.
 11. The method of claim 8 wherein the crosslinked fibers are present at a level from 40 to 60 percent of the total dry weight of the fiber weight in at least one ply of the paperboard.
 12. The method of claim 8 wherein the total starch level is 50 to 120 lb/t.
 13. The method of claim 8 wherein the total starch level is 60 to 100 lb/t.
 14. The method of claim 8 wherein the total starch level is 80 to 90 lb/t.
 15. A method for forming a paperboard comprising the steps of: forming a slurry of cellulose fibers comprising crosslinked fibers; adding a cationic starch and mixing with said slurry; adding an anionic starch subsequent to adding said cationic fixative; wherein, after each addition step the slurry ionic demand is less than zero; forming a fibrous web layer by withdrawing liquid from said slurry; drying said web to form a paperboard.
 16. The method of claim 15 wherein the anionic starch is added to the slurry before the cationic fixative.
 17. The method of claim 15 wherein said crosslinked fibers are present at a level from 25 to 80 percent of the total fiber weight in at least one ply of the paperboard.
 18. The method of claim 15 wherein the crosslinked fibers are present at a level from 40 to 60 percent of the total dry weight of the fiber weight in at least one ply of the paperboard.
 19. The method of claim 15 wherein the total starch level is 50 to 120 lb/t.
 20. The method of claim 15 wherein the total starch level is 60 to 100 lb/t.
 21. The method of claim 15 wherein the total starch level is 80 to 90 lb/t. 